Coated pipes are used, for example, in underground pipe construction. The diameters of the pipes are generally between 90 and 1200 mm, larger diameters also being possible. The pipes consist of a pipe as the carrier pipe, preferably made of steel, but other materials are also possible.
The pipe is preferably surrounded by insulation for mechanical and corrosion protection, the insulation generally being polyethylene as a coating material. For protection against thermal effects, intermediate layers made of rigid foam are also installed.
Jacket systems made of compact extruded thermoplastic materials on metal base pipes, also in combination with auxiliary layers, for example for improving adhesion, are predominantly used as mechanical or corrosion protection. However, these compact jacket systems are found to be highly resistant to bending. A flexible jacket structure is found to be particularly advantageous in particular in the case of flexible pipe systems which are intended for dynamic use. It is also an object of this invention to produce, instead of the compact jacket system produced hitherto, a jacket system having a cellular layer, in order thus to achieve significantly increased flexibility.
Jacket systems having a sandwich structure are also used for thermal insulation. The adhesion of such a sandwich system is of great importance for the pipe statics in the case of underground pipes carrying hot media, in order to ensure that the insulation provided for heat insulation performs its function fully over a long period of time. However, comprehensive tests have shown that such insulation, in particular rigid foam insulation, especially made of polyurethane foam, ages, in particular at high temperatures above approximately 120° C.
WO 2004/003423 A1 discloses an insulated pipe having one or more inner pipes and an insulating foam which surrounds the inner pipe. The pipe has a diffusion-regulating layer which is applied to the outside of the foam.
DE 10 2007 015 660 A1 describes a flexible heat-insulated conduit consisting of at least one medium-carrying inner pipe, an outer pipe surrounding the at least one inner pipe, and a polyurethane-foam-based heat-insulating layer, located between the at least one inner pipe and the outer pipe, in which hollow spheres are added to the polyurethane foam. The flexible polyurethane foam allows a certain amount of compression upon application of a force. The flexural strength of the conduit, in which the plastics inner pipe, the polyurethane foam and the plastics outer pipe form a composite, is thus decreased. The result is that, upon bending of the conduit pipe around tight radii, minimal or no breakages in the foam may be expected.
Furthermore, a flexible heat-insulated conduit is also known from EP 2 620 268 A1, for example. For accelerated hardening, the covering layer is cooled by a liquid coolant, in particular water.
DE 32 16 463 A1 relates to a method for producing a flexible district heating pipe comprising a central carrier pipe made of metal and foam insulation in a plastics jacket, which is extruded continuously as a tube from a thermoplastic material and then immediately cooled.
DE 93 10 530 U1 describes encasing a pipe in PUR foam, and JP H02-81 618 A describes encasing a pipe in foam in a coextrusion process using adhesion promoter with a smooth surface without an outer covering layer.
DE 195 07 110 A1 discloses encasing a pipe in PUR foam. The expanding foam is enclosed by a corrugated protective jacket.
WO 00/35 657 A1 relates to encasing a pipe in syntactic foam and to an extrusion head. The expanding foam follows the smooth protective jacket.
Furthermore, JP S54-123 167 A discloses an extrusion apparatus having a cooling device.
In a known production method, individual carrier pipes are provided with spacers and then a jacket pipe made of polyethylene is fitted. The polyurethane foam is then introduced at high speed into the cavity between the carrier pipe and the jacket pipe, which are situated in a slightly inclined plane, and then subsequently spreads out from top to bottom. A disadvantage of this method is that the polyurethane foam does not spread out in a laminar manner owing to the high injection speed and the long flow path. Turbulences form, which lead to cavitation and to density variations in the pipe. There are also pronounced local differences in the thermal conductivities and the mechanical properties.
In a continuous axial method, the carrier pipe is guided in a U-shaped aluminum foil and the aluminum foil is filled with polyurethane foam. The filled foil is then placed around the carrier pipe and sealed, so that the carrier pipe is jacketed by the polyurethane foam. The polyurethane foam is hardened in a calibration device. Finally, the pipe is jacketed by polyethylene.
In the continuous spiral method, the polyurethane foam is sprayed uniformly onto carrier pipes in a row next to one another while they rotate about their own axis. In a second step, a polyethylene jacket is applied in the form of a spiral to the pipe, which continues to rotate.
In the mixing head pulling technique, the polyurethane foam is introduced into the cavity between a carrier pipe and a jacket pipe by means of a mixing head, the mixing head being inserted into the cavity. The mixing head is pulled through the cavity, the cavity being filled thereby with polyurethane foam.
The high outlay associated with the production of insulation when a jacket pipe is used in addition to the carrier pipe and, in a further step, the insulating foam has to be introduced between the two pipes is found to be a disadvantage.